The functions of sleep constitute a genuine mystery in present day biology. Such functions can be investigated by preventing sleep from occurring. In rats, prolonged total or REM sleep deprivation lead to extreme sleepiness, weight loss, increase in food intake and metabolic rate, and death after 2-5 weeks. Decades of investigation have failed to reveal any substantial abnormality in peripheral organs or in the brain. Over the past few years, molecular approaches have been employed the to study the cellular correlates of sleep and waking. Several compounds have been shown to accumulate during waking and after short periods of sleep derivation and to return to basal levels during sleep, in line with homoeostatic models of sleep regulation. To evaluate the functional significance of these findings in the context of sleep homeostasis, new gene screening methods such as microarrays and differential display have been used to examine gene expression after both short-and long-term sleep deprivation. While most genes screened (approximately 10000) were not modified, the levels of brain arylsulfotransferase (AST) were increased by short periods of waking and even more so by prolonged sleep derivation. This enzyme is responsible for the inactivation of noradrenaline, which is released in the brain during waking and much less during NREM and REM sleep. The induction of AST may thus constitute a first indication of a homoeostatic response by the brain to the uninterrupted activity of the central noradrenergic system. This hypothesis will be tested by examining whether AST mRNA (1) progressively increases with the length of total sleep deprivation, increases after selective REM sleep derivation, and returns to normal after recovery sleep; (2) is accompanied by indices of increased central noradrenaline turnover; and (3) is reduced in animals with lesions of the central noradrenergic system. Finally, it will be established whether lesioned animals are more resistant to the harmful effects of sleep deprivation. These studies should provide insights into the molecular consequences of prolonged sleep deprivation and chronic insomnia and suggest new therapeutic approaches.